1. Field of the Invention
The present invention relates to a magnetoresistive element, and to a thin-film magnetic head, a head assembly and a magnetic disk drive each of which includes the magnetoresistive element.
2. Description of the Related Art
Performance improvements in thin-film magnetic heads have been sought as a real recording density of magnetic disk drives has increased. A widely used type of thin-film magnetic head is a composite thin-film magnetic head that has a structure in which a read head and a write head are stacked on a substrate. The read head has a magnetoresistive element (hereinafter, also referred to as an MR element) for reading, and the write head has an induction-type electromagnetic transducer for writing.
Examples of the MR element include a giant magnetoresistive (GMR) element utilizing a giant magnetoresistive effect, and a tunneling magnetoresistive (TMR) element utilizing a tunneling magnetoresistive effect. Read heads are required to have characteristics of high sensitivity and high output. As the read heads that satisfy such requirements, those incorporating spin-valve GMR elements or TMR elements have been mass-produced.
A spin-valve GMR element and a TMR element each typically include a free layer, a pinned layer, a spacer layer disposed between the free layer and the pinned layer, and an antiferromagnetic layer disposed on a side of the pinned layer farther from the spacer layer. The free layer is a ferromagnetic layer whose magnetization direction changes in response to a signal magnetic field. The pinned layer is a ferromagnetic layer whose magnetization direction is fixed. The antiferromagnetic layer is a layer that fixes the magnetization direction of the pinned layer by means of exchange coupling with the pinned layer. The spacer layer is a nonmagnetic conductive layer in a spin-valve GMR element, and is a tunnel barrier layer in a TMR element.
Examples of the GMR element include one having a current-in-plane (CIP) structure in which a current for use for detecting a signal magnetic field (hereinafter referred to as a sense current) is fed in the direction parallel to the planes of the layers constituting the GMR element, and one having a current-perpendicular-to-plane (CPP) structure in which the sense current is fed in a direction intersecting the planes of the layers constituting the GMR element, such as the direction perpendicular to the planes of the layers constituting the GMR element. The TMR element mentioned above also has the CPP structure.
A read head includes a pair of shields sandwiching the MR element. The distance between the two shields taken in a detection surface that receives a magnetic field to detect (i.e., a medium facing surface) is called a read gap length. Recently, with an increase in recording density, there have been increasing demands for a reduction in track width and a reduction in read gap length in read heads.
In a typical configuration of an MR element having the free layer, the pinned layer, the spacer layer and the antiferromagnetic layer, an end face of each of the free layer, the pinned layer, the spacer layer and the antiferromagnetic layer is exposed at the detection surface. In the MR element having such a configuration, it is difficult to reduce the read gap length because the antiferromagnetic layer is relatively great in thickness.
Attention has been recently given to carbon materials having a graphene structure, such as a graphene thin film and a carbon nanotube, because of their capability of conducting electrons over a long distance while conserving their spins. Here, the graphene structure refers to a structure in which a plurality of carbon atoms are bonded to one another in the form of a hexagonal mesh. The graphene thin film is a thin film consisting of a single atomic layer having a graphene structure or a plurality of atomic layers stacked each having a graphene structure. In particular, a thin film consisting only of a single atomic layer having a graphene structure is called graphene. The carbon nanotube is a tube composed of a film consisting of a single atomic layer having a graphene structure or a plurality of atomic layers each having a graphene structure.
A variety of MR elements using carbon materials having a graphene structure have been proposed. For example, U.S. Patent Application Publication No. 2004/0183110 A1, WO 2006/022859 A2, and JP-A 2004-221442 each disclose an MR element having two ferromagnetic layers that are opposed in the vertical direction and a carbon nanotube interposed between them.
WO 02/063693 A1 discloses an MR element in which two electrodes made of a magnetic metal are respectively provided on both ends of a multilayer carbon nanotube lying in the horizontal direction. European Patent No. 1052520 B1 discloses an MR element in which a channel region provided between first and second ferromagnetic regions is formed of a carbon nanotube, for example. JP-A 2007-335532 discloses an MR element in which two ferromagnetic electrodes are disposed on graphene formed on a silicon surface of a silicon carbide substrate.
U.S. Patent Application Publication No. 2004/0126304 A1 discloses a superconducting carbon nanotube that provides a great MR effect.
For the MR elements disclosed in U.S. Patent Application Publication No. 2004/0183110 A1, WO 2006/022859 A2 and JP-A 2004-221442, it is impossible to reduce the read gap length because these MR elements have such a structure that the spacer layer of a typical CPP-structure GMR element is replaced with a carbon nanotube.
The MR elements disclosed in WO 02/063693 A1, European Patent No. 1052520 B1 and JP-A 2007-335532 each have a structure in which two ferromagnetic layers serving as two electrodes are respectively provided on the horizontally-opposite sides of a carbon nanotube or graphene lying in the horizontal direction, and include no antiferromagnetic layer. These MR elements therefore allow a reduction in read gap length. However, it is difficult with these MR elements to achieve highly-accurate magnetic field detection because of the following reason. In these MR elements, the two ferromagnetic layers exhibit a change in magnetization direction in response to a signal magnetic field. WO 02/063693 A1 and European Patent No. 1052520 B1 teach configuring the two ferromagnetic layers to flip their magnetization directions at different magnetic field magnitudes so that the resistance of the MR element varies according to the magnitude of the signal magnetic field. However, for the MR elements that detect a signal magnetic field by such a method, the detectable magnitude range of the signal magnetic field is limited and the sensitivity to the signal magnetic field is reduced.